To enter your rental improvements, simply follow the directions to enter your rental income and expenses. At some point you'll come across the Here's[your property's] rental property info screen. Under Expenses / Assets (Depreciation), select Add expense or asset and follow the onscreen instructions.
Note: Although it doesn't seem logical, refinance fees and mortgage points are also entered in the Expenses / Assets (Depreciation) section. The IRS considers these amortizable intangibles and accounting rules dictate that those are to be depreciated instead of deducted as an expense.